To change the printing plates in sheet-fed offset printing machines, the used printing plate is usually removed first. This begins by opening a tension or clamping rail which normally grips the trailing or print end of the printing plate. With the plate cylinder slowly rotating backwards, this used printing plate is then conveyed out of the printing unit. After the tension or clamping rail gripping the leading or print start end of the plate has been opened, the printing plate can be extracted completely from the printing unit. The drawing on of a new printing plate thereupon takes place. Here the leading or print start end of this printing plate is inserted into the corresponding print-start tension rail. The plate cylinder is thereupon moved slowly in the forward direction of rotation and the printing plate is thus drawn around the cylinder and the print end of the printing plate is then inserted into the corresponding tension rail.
The work sequence briefly outlined above for changing a printing plate in a printing unit necessitates a correspondingly large number of manual handling operations on the devices of the plate cylinder, the printing unit and with the printing plate itself. For this reason, the changing of printing plates in a multicolor sheet-fed offset printing machine having a plurality of printing units takes up a correspondingly large amount of time.
So that some of the manual handling operations during the changing of printing plates can be avoided, there have already become known so-called semi-automatic printing plate changing systems, in which, in particular, the clamping, release, tensioning and loosening of the printing plates in the corresponding devices of the plate cylinders (tension rails) are executed automatically. Only the introduction and removal of the printing plates then still have to be carried out by an attendant. By means of additional introduction aids or other means of assistance, the necessary handling operations during the introduction of a new printing plate can be reduced and, thus, a saving of time in execution. An introduction aid of this kind is described under the designation "PPL" in the German periodical "Druckwelt" ("Printing World"), 4, 25th Feb. 1993, pages 28-29.
The apparatuses disclosed by EP 0,435,410 A2 and EP 0,432,660 A2 for the fully automatic change of printing plates in sheet-fed offset printing machines make any tending actions during the cycle of the printing plate changing operation unnecessary. Thus, there is provided on each printing unit a complete printing plate changing apparatus which consists essentially of a storage chamber for the supply and reception of one or more new or used printing plates respectively, and of a transport or conveying mechanism, by means of which the printing plates can be conveyed from the plate cylinder into the storage chamber and from the storage chamber onto the plate cylinder. According to EP 0,435,410 A2, the storage chamber can receive a plurality of printing plates for a number of consecutive runs, so that the printing machine can be prepared for a specific number of printing orders. According to this publication, the printing plate changing apparatus pivotably is arranged on a subsequent tower of a printing unit so as to be capable of being thrown onto the plate cylinder and thrown off from the latter.
According to the prior publications mentioned above, the apparatus located on each printing unit for the automatic printing plate change are fed with new printing plates by an attendant, for example during the printing run, or the used printing plates located in the storage chamber have to be extracted by an attendant. A correspondingly large number of handling operations have to be carried out in the individual storage chambers of the printing plate changing apparatuses assigned to the printing units. In the case of the storage chamber provided in EP 0,435,410 A2 for a specific number of new printing plates, a reorganization regarding the plate order or sequence for the printing machine sometimes results in an extremely arduous resorting or exchange with the printing plates already inserted into the storage chamber.
In the printing plate changing apparatuses disclosed by the publications mentioned, a general disadvantage is that a complete apparatus, consisting of a storage chamber and a transport/conveying mechanism for the printing plates, must be provided on each printing unit. Precisely where a large number of printing units is concerned, this results in a correspondingly high expense in terms of construction costs. A further disadvantage arises from the additional constructional space in the region between the printing units, which these previously known apparatuses require. Also, in the previously known printing plate changing systems, although there is provision for arranging these so as to be capable of being thrown onto and thrown off from the plate cylinder or for mounting them so as to be capable of being otherwise pivoted away, nonetheless, in the very confined space between the printing units of a sheet-fed offset printing machine of medium size, there is often considerable restriction in the freedom of movement of an attendant who has to carry out, for example, a rubber-blanket change, a change of an ink applicator roller or other service work. In addition to the disadvantages already mentioned, the previously known apparatuses allow only a fully automatic change of the printing plates and, in the event of a failure of or damage to a printing plate changer on one or more printing units, the printing plates have to be changed in the customary way by hand, if this is still at all possible.
EP 0,100,778 B1 discloses an apparatus for the automatic exchange of printing plates, which has a conveying unit for conveying the plate to a predetermined point on a printing station and a delivery unit for removing the plate from the conveying unit and for feeding this printing plate to the plate cylinder. This previously known apparatus provides special printing plates having, in particular, special holding mechanisms, such that the plate holding means form elastic units with the printing plates mountable on them. However, in conventional sheet-fed offset printing machines of the type under consideration here and in widespread use, special plate-holding means of this type, together with compatible printing plates, cannot be used.